The present invention relates to an air conditioner for a vehicle, which can prevent misassembly of doors and prevent damage of gears when one rack gear operates two sliding doors and increase the degree of mounting freedom of actuating means. The air conditioner for a vehicle, which includes an air-conditioning case having a heat exchanger, and a door disposed inside the air-conditioning case and sliding to adjust the degree of opening of an air passageway, includes: shafts having a first gear part geared with gear teeth formed on the door. One or more misassembly preventing parts are formed on the shafts and the first gear part.